Common image forming apparatus and developer device units are disclosed in, for example, Japanese Laid-Open Patent Publication No. 6-348117 (PTL 1), Japanese Laid-Open Patent Publication No. 2000-56547 (PTL 2), Japanese Laid-Open Patent Publication No. 2005-309265 (PTL 3), and Japanese Laid-Open Patent Publication No. 2009-168893 (PTL 4). These pieces of literature each disclose a single component development system, a dual component development system and a hybrid development system (hereinafter may be called an HBD development system) as development systems for visualizing (developing) an electrostatic latent image on a photoconductor.
In the dual component development system, toner and a carrier are used for a dual component developer. Stable charge is quickly given to toner by contact and friction with the carrier. In the dual component development system, the carrier is more likely to be scattered and adhered to the photoconductor as the system speed increases, so that image noise called carrier fogging is more likely to occur.
In the single component development system, only toner is used as a single component developer. Since no carrier exists in a developing portion, carrier fogging does not occur.
The HBD development system has a structure in which the single component development system and the dual component development system are combined together. First, in a section where toner is supplied and transported to a developer roller, supply toner is mixed with a carrier and stirred. Stable charge is quickly given to the toner. It is therefore advantageous in that less stress is imposed on the toner than in the single component development system in which charging is performed in a regulating portion. Furthermore, in the HBD development system, development is performed by forming a toner layer on a developer roller through use of an electric field. Therefore, carrier fogging does not occur even when the system speed increases since no carrier exists in the developing portion.
Japanese Laid-Open Patent Publication No. 2010-72468 (PTL 5) discloses an image forming apparatus in which two developer rollers are arranged opposite to one photoconductor (image carrier). When the single component development system or the HBD development system is adopted in this image forming apparatus, single component development with toner alone is carried out in a developing portion (a region between the surface of an image carrier on which an electrostatic charge image is formed and the developer rollers). By carrying out the single component development, carrier fogging, which is a disadvantage of the dual component development system, is prevented from occurring.
Moreover, the image forming apparatus in which two developer rollers are arranged opposite to one photoconductor can ensure a longer developing zone than in an image forming apparatus in which one developer roller is arranged opposite to one photoconductor. Even when the system speed of the image forming apparatus increases (e.g., in high-speed printing), a sufficient amount of toner can be transported to the photoconductor since the photoconductor and the developer rollers are opposed for a longer time.